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The group was drinking alcohol and passing a gun back and forth, pointing it at each other when the gun was fired, officials said.

Patrick O'Connor, 23, of Stoneham, was arrested in connection with the incident. He was charged with involuntary manslaughter, assault and battery with a dangerous weapon causing serious bodily injury and discharging a firearm.

"This is really just a tragic, tragic situation," Andrea Levy said, who is representing O'Connor. "It is still under investigation."

Atherton, who was also a National Guard veteran, was appointed to the department in February.

"He served with distinction in the Massachusetts Army National Guard, where he was a military police officer who deployed to Qatar in support of Operation Enduring Freedom from 2012 to 2013. He was awarded eight times during his National Guard career for his actions, including the receipt of the Army Commendation Medal,” Stoneham Fire Chief Matthew Grafton said.

Graton said Atherton was selfless and dedicated, and he overcame great tragedy as he lost both of his parents at a young age.

“I cannot help but feel that the best was yet to come for this young man, and we in the Stoneham Fire Department are still coming to terms with this loss,” he said.

O’Connor entered a not guilty plea Wednesday in Woburn District Court.

"The deceased and the defendant were video taping each other during the course of the evening, fooling around with the gun, pointing it at each other," Adrienne Lynch, of the Middlesex County District Attorney's Office said.

Police say O'Connor originally told them Atherton had shot himself but later admitted to shooting Atherton while the two were drinking alcohol and "fooling around with the gun." O'Connor told police he didn't think the gun was loaded.